It is known that epoxide resins, i.e., substances containing on average more than one 1,2-epoxide group per molecule, may be cured by reaction with various classes of substances to form cross-linked, infusible, insoluble products having valuable technical properties. Typical curing agents include aromatic polyamines.
Although these are useful curing agents, being employed chiefly to cure epoxide resins at room temperature or moderately elevated temperatures, they suffer from the drawback that they often cure the resin only slowly. The use of accelerators alleviates this drawback to some extent but the accelerating effect which these accelerators impart is relatively modest. We have now found that certain nitrates are potent accelerators for curing epoxide resins by aromatic polyamines.
U.S. Pat. No. 2,886,472 discloses that certain metal salts such as zinc fluoborate, magnesium perchlorate, potassium persulphate, zinc sulphate, magnesium fluoborate, copper fluoborate, copper persulphate, chromic nitrate, magnesium nitrate, and calcium phosphite act as curing agents for epoxide resins employed on textile materials, at high temperatures, typically 100.degree. to 200.degree. C. British Patent Specification No. 1,105,772 disclosed that curing of epoxide resins by aliphatic, cycloaliphatic, aromatic, or heterocyclic amines may be accelerated by alkaline earth metal nitrates, lead nitrate, or aluminium nitrate. In British Patent Specification No. 1,428,625 is disclosed a method of hardening epoxide resins with an amine hardener using, as accelerator, an amine-soluble salt of an alkali metal or ammonium, selected from their halides, nitrates, nitrites, thiocyanates, cyanates, and chlorates.
The accelerating effect we have discovered could not be predicted from an examination of the above mentioned patents, since other salts, such as alkali metal and ammonium nitrates and magnesium and other fluoborates, have only little accelerating effect.